The invention relates to a method of manufacturing a device comprising a film of an oxidic superconducting material in a desired pattern, which material comprises at least an alkaline earth metal, according to which method the component elements of the superconducting material are provided on a substrate in the form of a film, the alkaline earth metal being provided in the form of alkaline earth metal fluoride, after which the superconducting material is formed by means of a treatment at an increased temperature in the presence of water and oxygen.
In an article by P. M. Mankiewich et al. in Appl. Phys. Lett. 51(21), pages 1753-1755 (1987) a method is described in which a film of Y, Cu and BaF.sub.2 is provided on a substrate of SrTiO.sub.3 by means of vacuum deposition. BaF.sub.2 is used instead of, for example, Ba metal or BaO because it has a much smaller sensitivity to moisture. The film thickness is of from 100 to 500 nm. YBa.sub.2 Cu.sub.3 O.sub.7 is formed in an oxidation step in oxygen at 800.degree. to 920.degree. C. in 0.5 to 6 hours, which material exhibits superconductive behavior at temperatures below a critical temperature T.sub.c of from 85 to 92 K. Patterns are formed in the film thus obtained by means of scribing or, in a more accurate manner, by means of a positive photolithographic method which is carried out before or after the oxidation step and by means of which patterns having details of 2 .mu.m and larger can be formed. In accordance with a first embodiment the substrate is coated with a photoresist, exposed to light and developed according to a desired pattern after which a film of Y, Cu and BaF.sub.2 is provided by means of vacuum deposition. By dissolving the residual resist material the overlying parts of the film become detached ("lift-off"), after which the oxidation step can be carried out to form the superconducting material. In accordance with a second embodiment, first the starting materials are provided by means of vacuum deposition and the oxidation step is carried out, after which the film is coated with a photoresist which is exposed to light and developed in accordance with a desired pattern, after which the superconducting film is etched by means of a diluted acid.
In an article by A. M. Degantolo et al. in Appl. Phys. Lett. 52 (23), pages 1995-1997 (1988) a description is given of a method in which a film of Y.sub.2 O.sub.3, CuO and BaF.sub.2 is provided on a substrate of SrTiO.sub.3 by means of laser ablation of a target plate formed from a mixture of the said starting materials. Subsequently, an oxidation step in oxygen is carried out at 850.degree. C. for 1 hour. The oxygen flow contains a quantity of water in order to enable the conversion into YBa.sub.2 Cu.sub.3 O.sub.7. In the case of a film having a thickness of 0.63 .mu.m a critical temperature of 90 K. was observed.
In an article by C. E. Rice et al. in Appl. Phys. Lett. 52 (21), pages 1828-1830 (1988) a description is given of a method in which a film of CaF.sub.2, SrF.sub.2, Bi and Cu, to which Y and/or Pb may be added, is applied to a substrate of SrTiO.sub.3 or sapphire by means of vacuum deposition. Subsequently, an oxidation step in moist oxygen is carried out for 15 min. at 725.degree. C. and 5 min. at 850.degree. C. A Ca-Sr-Bi-Cu oxide or a mixture of such oxides is formed having a critical temperature of approximately 80 K. Patterns are formed in the superconducting film having a thickness of from 300 to 500 nm by means of a photolithographic process and etching using a diluted acid.
The photolithographic processes described hereinbefore, which are used to form a pattern in superconducting films have a number of disadvantages. In the "lift-off" method organic resist material is present before the starting materials are provided. Owing to this, the temperature range in which deposition can take place is limited. When the residual resist material is removed, for example by using acetone, there is the risk that carbon is incorporated in the superconducting film. Structures having details which are smaller than 2 .mu.m can hardly be formed by means of this method. The second method, which comprises etching, is slow because the starting materials must be dissolved in diluted acid. Moreover, this method is not accurate either because underetching takes place. Both methods have the disadvantage that the pattern formed has no flat surface which may be problematic when additional films must be provided to manufacture a device such as, for example, a semiconductor device.